Image encoding device and image encoding method

ABSTRACT

An encoding controller ( 106 ) generates and outputs a current skip signal (SK i ) to a switch circuit ( 108 ), and outputs a quantization parameter (Qp) for controlling a code amount after the encoding processing of a current VOP by an encoder ( 103 ) using a current scene change signal (SC i ), to an encoder ( 106 ). The current scene change signal (SC i ) is set on the basis of the current VOP, a previous scene change signal (SC i-1 ), and a previous skip signal (SK i-1 ).

TECHNICAL FIELD

The present invention relates to the encoding of a moving image, andmore specifically to an image encoding apparatus and an image encodingmethod for performing the encoding using a technique represented byITU-T Recommendation H.26x, ISO/IEC standard MPEG, and others. The imageencoding apparatus and the image encoding method according to thepresent invention are used in a device mounted in a cellular phone, forinstance.

BACKGROUND ART

MPEG-4 will be described below as an example. Generally, in an imageencoding method represented by MPEG-4, input image signal data iscompressed utilizing spatial and chronological correlations. The dataobtained utilizing the spatial and chronological compression isvariable-length encoded in a given sequence to generate a bit stream.

In MPEG-4, a whole display image (composite image) includes images(objects) of plural image series, so that an image plane of each imageseries at each display time is referred to as a video object plane(hereafter referred to as a “VOP”) and is distinguished from a frame ofMPEG-1 or MPEG-2. If a whole display image is formed from images of asingle image series, the VOP agrees with the frame.

A VOP has a luminance signal and a color-difference signal, and iscomposed of a plurality of macroblocks. A macroblock includes a 16 by 16matrix of pixels for the luminance signal. In the image encoding byMPEG-4, the amount of information is compressed by the spatialcompression, chronological compression, and other schemes in a unit ofeach of macroblocks. The spatial compression is performed by convertingthe signals from the time domain to the frequency domain using adiscrete cosine transform (hereafter referred to as a “DCT”), which is atype of orthogonal transformation, and then quantizing the convertedsignal. The chronological compression uses motion compensation.

Further, there are two methods of data compression in a unit of each ofthe VOPs: Spatial intra-picture coding (hereafter referred to as“intra-coding”) encodes a VOP using only spatial compression in the samepicture; and inter-picture coding (hereafter referred to as“inter-coding”) encodes a VOP using chronological compression using thecorrelation of pictures.

An image encoding apparatus must output a bit stream of a designatedcode amount in accordance with a given encoding parameter. The imageencoding apparatus must also control the amount of code generation inaccordance with the estimated amount of occupation in a buffer (avirtual buffer verifier, hereafter referred to as a “VBV buffer”) of thedecoding apparatus for receiving a bit stream, so that the VBV bufferwill not overflow nor underflow.

The amount of code generation is controlled in accordance with aquantization parameter, which is used to quantize a DCT coefficient setfor each macroblock contained in a VOP. Therefore, the amount of codegeneration is controlled in a unit of each of the VOPs. Generally, asthe quantization parameter increases, the amount of code generationdecreases; and as the quantization parameter decreases, the amount ofcode generation increases. That is, the amount of code generation andthe quantization parameter are in inverse proportion. Through the use ofthis property, the amount of code generation can be changed.

However, since the possible range of the quantization parameter islimited, it may be difficult to control the amount of code generationappropriately just in accordance with the quantization parameter, insome cases. So, if the amount of code generation is greater than atarget value, not all the VOPs are subjected to the encoding processing,and a VOP which is not encoded is generated, that is, a skip VOP, theencoding processing of which is skipped, is generated, therebysuppressing the total amount of code generation. On the other hand, ifthe amount of code generation is smaller than the target value,processing is performed to insert a redundant bit into a bit stream,thereby increasing the amount of code generation. The above-mentionedtechnique for suppressing the amount of code generation by skipping partof the VOP encoding processing is described in document 1 (JapanesePatent Kokai (Laid-Open) Publication No. 2002-262297, pages 4 to 7 andFIG. 3), for instance.

Further, document 2 (Japanese Patent Kokai (Laid-Open) Publication No.H6-54319, pages 4 to 5 and FIG. 2) describes a code amount controlmethod for detecting any scene change from an input signal and assigninga large code amount to an image immediately after the scene change sothat image degradation at a scene change can be reduced in an apparatusfor performing the encoding processing of an input image signal, forinstance.

An image encoding apparatus for performing the encoding processing inaccordance with the conventional MPEG-4 performs an encoding method forcontrolling the amount of code generation without performing theencoding processing for some VOPs, as described in document 1.Therefore, if a skip VOP, the encoding processing of which is skipped,is a VOP where a scene change is detected, the following problem arises.

Suppose that there are chronologically successive VOPs (respectivelydenoted as “VOP₁”, “VOP₂”, and “VOP₃” in chronological order) and that ascene change is detected in VOP₂, for instance. A greater code amountthan usual must be assigned to VOP₂. Otherwise, the image quality willbe degraded. However, if VOP₂ happens to be a skip VOP in order tosuppress the total amount of code generation, the encoding processing ofVOP₂ would not be performed. When the encoding processing of VOP₃ isperformed, information indicating that VOP₂ has a scene change has beenlost, and a normal code amount is assigned to the encoding processing ofVOP₃. This could degrade the image quality of VOP₃.

Further, if the encoding method as described in document 2 is performedto avoid image degradation by detecting a scene change and assigning agreater code amount to an image immediately after the scene change, thefollowing problem arises. When a scene change is detected in successiveVOPs, a great code amount is assigned successively. This can degrade theimage quality of a part other than the scene change or can cause a dropframe or the like. Therefore, an appropriate code amount cannot beassigned as a whole.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an image encodingapparatus and an image encoding method which can control the VOPencoding processing in such a manner that the image quality will not bedegraded even if a VOP where a scene change is detected becomes a skipVOP.

Further, another object of the present invention is to provide an imageencoding apparatus and an image encoding method which can assign anappropriate code amount to each VOP even if a scene change is detectedin successive VOPs.

The image encoding apparatus according to the present invention includesan encoder which performs encoding processing of input VOPssuccessively, thereby outputting a bit stream; a switch circuit whichallows the encoding processing by the encoder to be skipped in a unit ofeach of the VOPs; an encoding controller which generates an i-th skipsignal indicating whether or not the encoding processing of an i-th VOPis to be skipped and outputs the i-th skip signal to the switch circuit,where i is an integer not smaller than 1, the encoding controllergenerating a control signal for controlling a code amount after theencoding processing of the i-th VOP by the encoder using an i-th scenechange signal of the i-th VOP and outputting the control signal to theencoder; and a scene change signal generator which generates the i-thscene change signal to be input to the encoding controller on the basisof information of the i-th VOP and a VOP input prior to the i-th VOP andan (i-1)-th skip signal indicating whether or not the encodingprocessing of the (i-1)-th VOP input a period of N VOPs before the i-thVOP is skipped, where N is an integer not smaller than 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing a configuration of animage encoding apparatus in accordance with an embodiment of the presentinvention;

FIG. 2 is a flow chart showing the operation of encoding processing bythe image encoding apparatus in accordance with the embodiment of thepresent invention;

FIG. 3 is a diagram showing a configuration of a frame memory of theimage encoding apparatus in accordance with the embodiment of thepresent invention;

FIG. 4 is a flow chart showing the processing of setting a scene changesignal by the scene change detector of the image encoding apparatus inaccordance with the embodiment of the present invention;

FIGS. 5A and 5B are diagrams showing a relationship between a scenechange signal and a skip signal in the image encoding apparatus inaccordance with the embodiment of the present invention; and

FIGS. 6A and 6B are diagrams showing a relationship among the amount ofchange between VOPs, change amount detection information, a skip signal,and a scene change signal in the image encoding apparatus in accordancewith the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An image encoding apparatus in accordance with an embodiment of thepresent invention determines a current scene change signal for a currentVOP, by not only referencing a signal indicating whether or not theamount of change in information (change in a characteristic amount suchas luminance signal and color-difference signal) between the currentVOP, the encoding processing of which is performed, and a previous VOP,which exists a period of N VOPs (N is an integer not smaller than 1)before the current VOP is greater than a predetermined value but alsoreferencing a previous scene change signal indicating whether or not theprevious VOP is a scene change VOP and a previous skip signal indicatingwhether or not the encoding processing of the previous VOP is performed.The image encoding apparatus sets a target code amount in accordancewith the current scene change signal, and sets a quantization parameterrequired to set the target code amount.

The present embodiment will be described by taking an example where N is1, but N is not limited to 1. In the following description, the currentVOP may also be expressed as “VOP_(i)”, where i is an integer notsmaller than 1, and the previous VOP may also be expressed as“VOP_(i-1)”. Further, the current skip signal may also be expressed as“SK_(i)”, and the previous skip signal may also be expressed as“SK_(i-1)”. In addition, the current scene change signal may also beexpressed as “SC_(i)”, and the previous scene change signal may also beexpressed as “SC_(i-1)”.

When N is 1, the image encoding apparatus appropriately performs theencoding processing of the current VOP by referencing not onlyinformation indicating whether or not the previous VOP is a scene changeVOP but also the information indicating whether or not the encodingprocessing of the previous VOP is performed (that is, whether or not theencoding processing of the previous VOP is skipped), when determiningthe current scene change signal SC_(i).

In the following description, if the amount of change in informationbetween the previous VOP and the current VOP is greater than apredetermined value, it is said that the current VOP is a scene changeVOP. If the amount of change in information between the previous VOP andthe current VOP is not greater than the predetermined value, it is saidthat the current VOP is not a scene change VOP.

Further, if the amount of change in information between the previous VOPand the current VOP is greater than a predetermined value and if theamount of change in information between a VOP input N VOPs before theprevious VOP (hereafter referred to as a “VOP preceding the previous VOPor twice previous VOP” or “VOP_(i-2)”) and the previous VOP is notgreater than a predetermined value, it is said that the current VOP is asingle isolated scene change VOP. Furthermore, if the amount of changein information between the previous VOP and the current VOP is greaterthan a predetermined value and if the amount of change in informationbetween the twice previous VOP and the previous VOP is greater than apredetermined value, it is said that the current VOP is a continuedscene change VOP.

The image encoding apparatus in accordance with the embodiment of thepresent invention detects whether or not the current VOP is a scenechange VOP and, if the current VOP is a scene change VOP, furtherdetects whether or not the current VOP is a continued scene change VOP.On the bases of such detections, whether the current mode is a modewhere the current VOP is detected as a single isolated scene change VOPor a mode where the current VOP is detected as a continued scene changeVOP or a mode where the current VOP is not a scene change VOP can bedetected. Then, the image encoding apparatus in accordance with theembodiment of the present invention sets a quantization parameter and atarget code amount after the encoding of the current VOP, in accordancewith the detected mode, and performs appropriate encoding processing ofall of the successively input VOPs.

The image encoding apparatus and the image encoding method of thepresent invention will now be described in further detail with referenceto drawings.

FIG. 1 is a block diagram showing a configuration of an image encodingapparatus 100 in accordance with the embodiment of the presentinvention. As shown in FIG. 1, the image encoding apparatus 100 receivesan image signal transmitted by wired communication or wirelesscommunication, as a VOP. The image encoding apparatus 100 divides theinput VOP into macroblocks, performs encoding processing, and outputs abit stream.

As shown in FIG. 1, the image encoding apparatus 100 includes a framememory 101 which stores an input VOP temporarily, a change amountdetector 102 which detects the amount of change in information among theVOPs stored in the frame memory 101, a switch circuit 108, an encoder103 which encodes the VOP input through the switch circuit 108 in a unitof each of macroblocks, a buffer 104 which temporarily stores a bitstream output from the encoder 103, a code amount detector 105 whichcounts the amount of code generation of each VOP from the bit streamstored in the buffer 104, and an encoding controller 106 determining aquantization parameter for the encoding performed by the encoder 103,for instance, on the basis of the scene change signal output from ascene change detector 107.

The image encoding apparatus 100 further includes the scene changedetector 107 which generates a current scene change signal SC_(i) inaccordance with the amount of change in information among VOPs (amountof change in a characteristic amount, for instance) detected by thechange amount detector 102, a previous skip signal SK_(i-1) output fromthe skip signal holder 109, a previous scene change signal SC_(i-1)output from the scene change signal holder 110, and the like; a scenechange signal holder 110 which temporarily holds a scene change signaloutput from the scene change detector 107; and a skip signal holder 109which temporarily holds a skip signal output from the encodingcontroller 106. The skip signal is a current skip signal SK_(i) when itis input to the skip signal holder 109, and becomes a previous skipsignal SK_(i-1) when it is output to the scene change detector 107(after a lapse of time from when it is input to the skip signal holder109).

Further, the frame memory 101, the change amount detector 102, the scenechange detector 107, the skip signal holder 109, and the scene changesignal holder 110 constitute a scene change signal generator 111 forgenerating and outputting a current scene change signal SC_(i).

Furthermore, the encoding controller 106 further includes a skipcontroller 106 a which outputs a current skip signal SK_(i), whichindicates whether or not the encoding processing of the current VOP isskipped, and a quantization parameter controller 106 b which sets atarget code amount in the encoding processing of the current VOP and aquantization parameter.

FIG. 2 is a flow chart showing the operation of VOP encoding processingperformed by the image encoding apparatus 100 in accordance with theembodiment of the present invention. With reference to FIG. 2, anoverview of the VOP encoding processing will be described.

As shown in FIG. 2, the change amount detector 102 first detects theamount of change D_(i) (for instance, amount of change in characteristicamounts such as luminance signal) between the current VOP and theprevious VOP input to the image encoding apparatus 100 (step S1).

Next, the scene change detector 107 sets the current scene change signalSC_(i) on the basis of the amount of change D_(i) detected in step S1and the previous scene change signal SC_(i-1) (step S2). When theprevious scene change signal SC_(i-1) is judged, whether or not theencoding processing of the previous VOP has been skipped is considered.When the previous scene change signal SC_(i-1) is judged, it is alsoconsidered whether the previous VOP is a single isolated scene changeVOP or is a continued scene change VOP or is not a scene change VOP.Accordingly, whether or not the previous VOP is a continued scene changeVOP can be detected from the previous scene change signal SC_(i-1).

This embodiment uses three types of scene change signals, for instance:a signal when the current VOP is detected as not being a scene changeVOP (signal value ‘0’), a signal when the current VOP is detected asbeing a single isolated scene change VOP (signal value ‘1’), and asignal when the current VOP is detected as being a continued scenechange VOP (signal value ‘2’).

It is judged whether or not the encoding processing of the current VOPis skipped (that is, whether or not the encoding processing of thecurrent VOP is performed) on the basis of the code amount of theprevious VOP and the size of the free space in the VBV buffer forreceiving a bit stream output from the image encoding apparatus 100; andthe current skip signal SK_(i) for indicating whether or not theencoding processing of the current VOP is skipped is set (step S3). Thisembodiment uses two types of skip signals: a signal when it is judgedthat the encoding processing of the current VOP is performed (signalvalue ‘0’) and a signal when it is judged that the encoding processingof the current VOP is not performed and the encoding processing isskipped (signal value ‘1’).

If it is judged that the encoding processing of the current VOP is notperformed (that is, the current VOP is a skip VOP) on the basis of thecurrent skip signal SK_(i) set in step S3 (that is, if the signal valueof the current skip signal SK_(i) is ‘1’), the image encoding apparatus100 does not perform the encoding processing of the current VOP andterminates the processing of the current VOP judged as being a skip VOP(step S4). In step S4, if the current VOP is judged as being a VOP (thatis, a VOP which is not a skip VOP), the encoding processing of which isperformed, that is, if the signal value of the current skip signalSK_(i) is ‘0’, the processing of the image encoding apparatus 100proceeds to next step S5.

The case where it is judged that the current VOP is a VOP, the encodingprocessing of which is performed, includes the following three cases,which will be described later: the case where the current VOP is asingle isolated scene change VOP (that is, the case where the signalvalue of the current scene change signal SC_(i) scene change VOP (thatis, the case where the signal value of the current scene change signalSC_(i) is ‘2’), or the case where the current VOP is not a scene changeVOP (that is, the case where the signal value of the current scenechange signal SC_(i) is ‘0’).

The encoding controller 106 sets a target code amount of the encodingprocessing (step S5) in such a way that the code amount after encodingprocessing when the current VOP is a single isolated scene change VOP(that is, when the signal value of the current scene change signalSC_(i) is ‘1’) becomes greater than the code amount after encodingprocessing when the current VOP is not a scene change VOP (that is, whenthe signal value of the current scene change signal SC_(i) is ‘0’) orwhen the current VOP is a continued scene change VOP (that is, when thesignal value of the current scene change signal SC_(i) is ‘2’).

The encoding controller 106 sets a target code amount of the encodingprocessing (step S5) in such a way that the code amount after encodingprocessing when the current VOP is a continued scene change VOP (thatis, when the signal value of the current scene change signal SC_(i) is‘2’) does not become greater than the code amount after encodingprocessing when the current VOP is a single isolated scene change VOP(that is, when the signal value of the current scene change signalSC_(i) is ‘1’).

The quantization parameter Qp of each macroblock is determined (step S6)on the basis of the target code amount set in step S5, and the actualencoding processing of the current VOP is performed (step S7).

The operation of the VOP encoding processing of the image encodingapparatus 100 will next be described in further detail, with referenceto FIG. 1 to FIG. 3. FIG. 3 is a conceptual diagram showing aconfiguration of the frame memory 101 of the image encoding apparatus100 in accordance with this embodiment.

First, a VOP input from a host control system (not shown) whichexercises centralized control over the blocks of the image encodingapparatus 100 of this embodiment is stored in the frame memory 101temporarily. The frame memory 101 includes two areas, as shown in FIG.3: a first area 101 a for storing the data of the (i-1)-th VOP, where iis a positive integer, and a second area 101 b for storing the data ofthe i-th VOP. Accordingly, the data of two successive VOPs are held inthe frame memory 101.

Next, the change amount detector 102 obtains the amount of change ininformation between two successive VOPs stored in the frame memory 101(step S1) from the sum of the absolute values of differences among thepixels with regard to the luminance signal, for instance. The switchcircuit 108 is turned on or off, on the basis of the skip signal SK_(i)from the encoding controller 106, which will be described later. The VOPinput through the switch circuit 108 to the encoder 103 is encoded in aunit of each of macroblocks by the encoder 103, in accordance with aquantization parameter Qp and a target code amount set by the encodingcontroller 106. The encoder 103 outputs a bit stream subjected to theencoding processing to the buffer 104. The bit stream output from theencoder 103 is stored in the buffer 104 temporarily and then output.

The bit stream stored in the buffer 104 is output in accordance with acontrol signal from the host control system described above. The outputfrom the buffer 104 becomes the output of the image encoding apparatus100. The bit stream stored in the buffer 104 is input to the code amountdetector 105 as well.

The code amount detector 105 detects the magnitude of the code amount ofthe bit stream input from the buffer 104 to the code amount detector 105(that is, the code amount after the encoding processing of the currentVOP by the encoder 103) and outputs the result of detection to theencoding controller 106.

The scene change detector 107 sets the current scene change signalSC_(i), on the basis of the amount of change in information between thecurrent VOP and the previous VOP output from the change amount detector102 (amount of change in a characteristic amount), the scene changesignal of the previous VOP output from the scene change signal holder110 (that is, the previous scene change signal SC_(i-1)), and the skipsignal of the previous VOP output from the skip signal holder 109 (thatis, the previous skip signal SK_(i-1)); and outputs the set currentscene change signal SC_(i) to the encoding controller 106 (step S2). Thesetting of the current scene change signal SC_(i) will be describedlater in further detail.

The encoding controller 106 first determines whether or not the encodingprocessing of the current VOP is performed, and sets the current skipsignal SK_(i) in accordance with the determination (step S3). After thecurrent skip signal SK_(i) is set, the target code amount of the currentVOP is set; the quantization parameter Qp is set in accordance with thetarget code amount; the current skip signal SK_(i) is output to theswitch circuit 108 and the skip signal holder 109; and the quantizationparameter is output to the encoder 103. The switch circuit 108 is turnedon or off in accordance with the current skip signal SK_(i).

Whether or not the encoding processing of the current VOP is performedis determined by the skip controller 106 a in the encoding controller106, from the code amount of the bit stream corresponding to theprevious VOP output from the code amount detector 105 and the size ofthe free space in the VBV buffer which is not shown. That is, whether ornot the encoding processing of the current VOP is performed isdetermined depending on whether or not a problem occurs in the VBVbuffer for receiving the bit stream if the code amount of the bit streamcorresponding to the current VOP matches the code amount of the bitstream corresponding to the previous VOP after the encoding processingof the current VOP is performed. If the encoding processing of theprevious VOP is skipped, the code amount of the bit stream correspondingto a VOP prior to the previous VOP (a VOP existing N VOPs before theprevious VOP, expressed as a “twice previous VOP” or “VOP_(i-2)”, forinstance) is referenced. The code amount to be referenced when theencoding processing of the previous VOP is skipped may also be apredetermined fixed value.

In step S4, if it is judged that the encoding processing of the currentVOP is not performed and the encoding processing is skipped, the skipcontroller 106 a sets the signal value of the current skip signal SK_(i)to ‘1’. When the signal value of the current skip signal SK_(i) is ‘1’,the switch circuit 108 is turned off, and the signal value ‘1’ of thecurrent skip signal SK_(i) is input to the skip signal holder 109. Onthe other hand, if it is judged that the encoding processing of thecurrent VOP is performed, the skip controller 106 a sets the signalvalue of the current skip signal SK_(i) to ‘0’ in order to indicate thatthe encoding processing of the current VOP is performed, and the signalvalue ‘0’ of the current skip signal SK_(i) is input to the skip signalholder 109.

In step S5, when the signal value of the current skip signal SK_(i) is‘0’, the quantization parameter controller 106 b in the encodingcontroller 106 sets a target encoding value for encoding, on the basisof the current scene change signal SC_(i) input from the scene changedetector 107. The scene change signal has three modes as describedlater: a mode in which the current VOP is detected as not being a scenechange VOP, a mode in which the current VOP is detected as being asingle isolated scene change VOP, and a mode in which the current VOP isdetected as being a continued scene change VOP.

When the current VOP is detected as being a continued scene change VOP,if the target encoding value set in a similar manner to the case wherethe current VOP is detected as being a continued scene change VOP, agreat code amount is assigned to successive VOPs, which can degrade theimage quality of a portion other than the scene change portion.Therefore, the encoding controller 106 sets the target code amount whenthe current VOP is detected as being a continued scene change VOP to alevel lower than the target code amount when the current VOP is detectedas being a single isolated scene change VOP; and the quantizationparameter controller 106 b sets a quantization parameter Qp of theencoding processing on the basis of the set target code amount (stepS6). The encoding controller 106 turns on the switch circuit 108,outputs a quantization parameter Qp to the encoder 103, and inputs thesignal value ‘0’ of the current skip signal SK_(i) to the skip signalholder 109.

If it is detected that the current VOP is a single isolated scene changeVOP, the quantization parameter controller 106 b sets a greater targetcode amount than when the current VOP is detected as being a continuedscene change VOP or when the current VOP is detected as not being ascene change VOP, and quantization parameter Qp of the encodingprocessing is set on the basis of the set target code amount (step S6).The encoding controller 106 then turns on the switch circuit 108,outputs a quantization parameter Qp to the encoder 103, and inputs thesignal value ‘0’ of the current skip signal SK_(i) to the skip signalholder 109. Therefore, an image display apparatus using the imageencoding apparatus 100 of the embodiment can display an image with astable quality even at a scene change.

If it is detected that the current VOP is not a scene change VOP, thequantization parameter controller 106 b sets a smaller target codeamount than when the current VOP is detected as being a single isolatedscene change VOP, and sets quantization parameter Qp of the encodingprocessing on the basis of the set target code amount (step S6). Theswitch circuit 108 is then turned on, quantization parameter Qp isoutput to the encoder 103, and the signal value ‘0’ of the current skipsignal SK_(i) is input to the skip signal holder 109.

A target encoding value is set for each mode, and a quantizationparameter Qp is set accordingly, in the encoding processing of thecurrent VOP, so that the code amount of the encoding processingperformed by the encoder 103 can be controlled to an appropriate levelas a whole.

FIG. 4 is a flow chart showing the processing of setting the currentscene change signal SC_(i) by the image encoding apparatus 100. Further,FIGS. 5A and 5B are diagrams showing the current scene change signalSC_(i) corresponding to the previous skip signal SK_(i-1) and theprevious scene change signal SC_(i-1) in the image encoding apparatus100.

The method of setting a scene change signal by the scene change detector107 of the image encoding apparatus 100 will now be described withreference to FIG. 4 and FIGS. 5A and 5B. FIG. 5A shows the processing ofsteps S12 to S18 in FIG. 4, and FIG. 5 B shows the processing of stepsS19 to S21 in FIG. 4.

The scene change detector 107 detects whether or not the current VOP isa scene change VOP. If the current VOP is a scene change VOP, the scenechange detector 107 detects whether the current VOP is a continued scenechange VOP or a single isolated scene change VOP.

As shown in FIG. 4, in step S11, the scene change detector 107 judgeswhether or not the amount of change in information between successiveVOPs, that is, the amount of change D_(i) in information between thecurrent VOP and the previous VOP, is greater than a predetermined amountof change in information. If it is judged in step S11 that the amount ofchange D_(i) is greater than the predetermined value, the current VOP isjudged as being a scene change VOP, and the processing of the scenechange detector 107 proceeds to step S12. If it is judged in step S11that the amount of change D_(i) is not greater than the predeterminedvalue, the current VOP is judged as not being a scene change VOP, andthe processing of the scene change detector 107 proceeds to step S19. Ashas been described earlier, the scene change detector 107 receives theamount of change detected by the change amount detector 102. Thedetected amount of change is, for instance, the sum of absolute valuesof differences among the pixels of successive VOPs with regard to theluminance signal.

When the processing proceeds to step S12, it is judged whether or notthe previous VOP is a skip VOP, the encoding processing of which is notperformed, on the basis of the previous skip signal SK_(i-1). If theprevious VOP is judged as being a skip VOP (that is, if the signal valueof the previous skip signal SK_(i-1) is ‘1’), the processing proceeds tostep S16. If it is judged in step S12 that the previous VOP is not askip VOP (that is, if the signal value of the previous skip signalSK_(i-1) is ‘0’), the processing proceeds to step S13. If the previousVOP is a skip VOP, a relationship between the twice previous VOP, whichexists N VOPs prior to the previous VOP, and the current VOP is needed.To detect the state of the twice previous VOP, the relationship with theVOP existing N VOPs prior to the twice previous VOP (hereafter expressedas a “VOP preceding VOP preceding the previous VOP or three timesprevious VOP” or “VOP_(i-3)”) is needed.

In step S12, if the signal value of the previous skip signal SK_(i-1) isnot ‘0’ but ‘1’, the previous VOP is a skip VOP, so that the processingof the scene change detector 107 proceeds to step S16.

In step S16, it is detected whether or not the signal value of theprevious scene change signal SC_(i-1) is ‘2’. That is, it is detectedwhether the previous VOP is a continued scene change VOP or the previousVOP is the other VOPs (the latter is a case where the signal value ofthe previous scene change signal SC_(i-1) is ‘0’ or ‘1’). Because it isdetected in step S12 that the previous VOP is a skip VOP, therelationship with the states of the previous VOP, the twice previousVOP, and the three times previous VOP is needed to judge the state ofthe current VOP.

If the signal value of the previous scene change signal SC_(i-1) is ‘2’,that is, if the previous VOP is a continued scene change VOP, in stepS16:

(1a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is greater than a predeterminedvalue;

(1b) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isgreater than a predetermined value;

(1c) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is greaterthan a predetermined value.

Accordingly, if just the VOP, the encoding processing of which isperformed, is considered,

(2a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is greater than thepredetermined value; and

(2b) The amount of change in information between the twice previous VOPand the current VOP is greater than a predetermined value.

So, the current VOP is considered to be a continued scene change VOP,and the signal value of the current scene change signal SC_(i) is set to‘2’ (step S17).

If the signal value of the previous scene change signal SC_(i-1) is ‘1’,that is, if the previous VOP is a single isolated scene change VOP, instep S16,

(3a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is not greater than apredetermined value;

(3b) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isgreater than a predetermined value; and

(3c) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is greaterthan a predetermined value.

Accordingly, if just the VOPs, the encoding processing of which isperformed, are considered,

(4a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is not greater than apredetermined value; and

(4b) The amount of change in information between the twice previous VOPand the current VOP is greater than a predetermined value.

Therefore, the current VOP is judged to be a single isolated scenechange VOP, and the signal value of the current scene change signalSC_(i) is set to ‘1’ (step S18).

If the signal value of the previous scene change signal SC_(i-1) is ‘0’,that is, if the previous VOP is neither a single isolated scene changeVOP nor a continued scene change VOP, in step S16,

(5a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is not greater than apredetermined value;

(5b) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isnot greater than a predetermined value; and

(5c) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is greaterthan a predetermined value.

Accordingly, if just the VOPs, the encoding processing of which isperformed, are considered,

(6a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is not greater than apredetermined value; and

(6b) The amount of change in information between the twice previous VOPand the current VOP is greater than a predetermined value.

Therefore, the current VOP is treated as a single isolated scene changeVOP, and the signal value of the current scene change signal SC_(i) isset to ‘1’ (step S18).

The current scene change signal SC_(i) has a mode detected as being acontinued scene change as well as a mode detected as being a singleisolated scene change and a mode detected as not being a scene change,as described above, and the target code amount and the quantizationparameter in the mode detected as being a continued scene change are setso that the code amount of the bit stream becomes smaller than the onein the mode detected as being a single isolated scene change, and theencoding processing is performed. In this case, an appropriate codeamount can be assigned to each VOP even if there are successive scenechange VOPs, and appropriate encoding processing can be consequentlyperformed as a whole.

The code amount of the bit stream generated by the encoding processingof the VOP is controlled by referencing the skip signal and the scenechange signal of the previous VOP and detecting a scene change of thecurrent VOP even if the skip VOP is a scene change VOP, so that detectedinformation is appropriately used and a stable image quality can beobtained.

A case in which the previous VOP is judged as being a VOP, the encodingprocessing of which is performed, in step S12 will next be described. Inthis case, the processing of the image encoding apparatus 100 proceedsfrom step S12 to step S13. In step S13, it is judged whether theprevious scene change signal SC_(i-1) is ‘1’ or ‘2’ (that is, whetherthe previous VOP is a scene change) or ‘0’ (that is, whether theprevious VOP is not a scene change). It is judged in step S11 that theamount of change in information between the previous VOP and the currentVOP is greater than a predetermined value, and the encoding processingof the previous VOP is performed, so that just the relationship betweenthe twice previous VOP and the previous VOP must be checked in step S13.

If the signal value of the previous scene change signal SC_(i-1) is ‘0’,that is, if the previous VOP is neither a single isolated scene changeVOP nor a continued scene change VOP in step S13:

(7a) The amount of change in information between the twice previous VOPand the previous VOP is not greater than a predetermined value; and

(7b) The amount of change in information between the previous VOP andthe current VOP is greater than a predetermined value.

Accordingly, the current VOP is judged as being a single isolated scenechange VOP, and the signal value of the current scene change signalSC_(i) is set to ‘1’ (step S14).

On the other hand, if the signal value of the previous scene changesignal SC_(i-1) is ‘1’, that is, if the previous VOP is a singleisolated scene change VOP, in step S13:

(8a) The amount of change in information between the twice previous VOPand the previous VOP is greater than a predetermined value; and

(8b) The amount of change in information between the previous VOP andthe current VOP is greater than a predetermined value.

Accordingly, the current VOP is judged as being a continued scene changeVOP, and the signal value of the current scene change signal SC_(i) isset to ‘2’ (step S15).

If the signal value of the previous scene change signal SC_(i-1) is ‘2’,that is, if the previous VOP is a continued scene change VOP, in stepS13:

(9 a) The amount of change in information between the twice previous VOPand the previous VOP is greater than a predetermined value; and

(9b) The amount of change in information between the previous VOP andthe current VOP is greater than a predetermined value.

Accordingly, the current VOP is judged as being a continued scene changeVOP, and the signal value of the current scene change signal SC_(i) isset to ‘2’ (step S15).

If the current scene change signal SC_(i) has a mode detected as being acontinued scene change as well as a mode detected as being a singleisolated scene change and a mode detected as not being a scene change,as described above, the target code amount and the quantizationparameter are judged so that the code amount of the bit stream in themode detected as being a continued scene change becomes smaller thanthat in the mode detected as being a single isolated scene change, andthe encoding processing is performed. An appropriate code amount can beassigned to each VOP even if there are successive scene changes, andappropriate encoding processing can be consequently performed as awhole.

An example in which the amount of change in information betweensuccessive VOPs is not greater than a predetermined value will next bedescribed. The following description corresponds to the processing ofsteps S11 to S19 in FIG. 4 and relates to FIG. 5B.

In this case, it is judged first whether or not the previous VOP is askip VOP (step S19). This is because, if the previous VOP is a skip VOP,the information of the previous VOP must be held.

In step S19, if the signal value of the previous skip signal SK_(i-1) is‘0’, that is, if the previous VOP is a VOP, the encoding processing ofwhich is performed, the processing proceeds to step S20. It is detectedin step S11 that:

(10a) The amount of change in information between the previous VOP andthe current VOP is not greater than a predetermined value. Therefore,the relationship between the previous VOP and the current VOP does notchange even if the amount of change in information between the twiceprevious VOP and the previous VOP is greater than a predetermined valueor is not greater than the predetermined value. In this case, it isjudged that the current VOP is neither a single isolated scene changeVOP nor a continued scene change VOP, and the signal value of thecurrent scene change signal SC_(i) is set to ‘0’ (step S20).

On the other hand, if the signal value of the previous skip signalSK_(i-1) is ‘1’ in step S19, that is, if the previous VOP is a skip VOP,the relationship between the twice previous VOP and the previous VOP isneeded.

If the signal value of the previous scene change signal SC_(i-1) is ‘0’,that is, if the previous VOP is neither a single isolated scene changeVOP nor a continued scene change VOP, the amount of change ininformation between the twice previous VOP and the previous VOP, theencoding processing of which is skipped, is not greater than apredetermined value. Accordingly:

(11a) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isnot greater than a predetermined value; and

(11b) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is notgreater than a predetermined value.

Therefore, the current VOP is judged as neither being a single isolatedscene change VOP nor being a continued scene change VOP, and the signalvalue of the current scene change signal SC_(i) is set to ‘0’ (step S21)If the signal value of the previous scene change signal SC_(i-1) is ‘1’in step S19, that is, if the previous VOP is a single isolated scenechange VOP:

(12a) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isgreater than a predetermined value; and

(12b) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is notgreater than a predetermined value.

Therefore:

(13a) The amount of change in information between the twice previous VOPand the current VOP is greater than a predetermined value. So, thecurrent VOP is judged as being a single isolated scene change VOP, andthe signal value of the current scene change signal SC_(i) is set to ‘1’(step S21).

If the signal value of the previous scene change signal SC_(i-1) is ‘2’in step S19, that is, if the previous VOP is a continued scene changeVOP:

(14a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is greater than a predeterminedvalue;

(14b) The amount of change in information between the twice previous VOPand the previous VOP, the encoding processing of which is skipped, isgreater than a predetermined value; and

(14c) The amount of change in information between the previous VOP, theencoding processing of which is skipped, and the current VOP is notgreater than a predetermined value.

Accordingly:

(15a) The amount of change in information between the three timesprevious VOP and the twice previous VOP is greater than a predeterminedvalue; and

(15b) The amount of change in information between the twice previous VOPand the current VOP is greater than a predetermined value. Therefore,the current VOP is judged as being a continued scene change VOP, and thesignal value of the current scene change signal SC_(i) is set to ‘2’(step S21).

That is, in any case, the signal value of the previous scene changesignal SC_(i-1) is held, and the signal value of the current scenechange signal is set to the same signal value as the previous scenechange signal SC_(i-1).

If a skip VOP is a scene change VOP, the skip signal and the scenechange signal of the previous VOP are referenced to detect a scenechange, so that detected information can be appropriately used, codeamount control can be performed as expected, and a stable image qualitycan be obtained, as has been described above.

FIGS. 6A and 6B are diagrams for describing the relationship among theamount of change between the current VOP and the previous VOP, the skipsignal, and the scene change signal in the image encoding apparatus 100.To be more specific, FIGS. 6A and 6B show the following items (1) to(4):

The item (1) shows whether the amount of change between the current VOPand the previous VOP is greater or is not greater than a predeterminedvalue for judging whether or not there is a scene change, with regard toeach VOP of from VOP₁ to VOP₅.

The item (2) shows the change amount detection information based on theamount of change between the current VOP and the previous VOP.

The item (3) shows the skip signal indicating whether or not theencoding processing of each VOP is performed.

The item (4) shows the set value of the current scene change signalSC_(i) of each VOP, based on the change amount detection informationbased on the amount of change between the current VOP and the previousVOP, the previous skip signal SK_(i-1), and the previous scene changesignal SC_(i-1).

FIG. 6A shows an example in which a single isolated scene change VOP isa skip VOP. In FIG. 6A, the amount of change in information between VOP₂and the preceding VOP₁ is not greater than a predetermined value. Inthis case, the change amount detection information is set to ‘0’, whichindicates that the amount of change is not greater than a predeterminedvalue. Accordingly, the scene change signal of VOP₂ is set to ‘0’, whichindicates that VOP₂ is not a scene change VOP. On the other hand, ifVOP₂ is judged as being a VOP, the encoding processing of which isperformed, as a result of code amount control, the signal value of theskip signal of VOP₂ is set to ‘0’, which indicates that the VOP is not askip VOP.

Next, if the amount of change in information between VOP₂ and VOP₃ isgreater than a predetermined value, VOP₃ is detected as being a scenechange VOP and the signal value of the change amount detectioninformation is set to ‘1’. Suppose that VOP₃ is judged as being a skipVOP, the encoding processing of which is not performed, as a result ofcode amount control (a part surrounded by a circle depicted by a brokenline in FIG. 6A). In accordance with the setting method described in theflow chart of FIG. 4, the amount of change in information between VOP₃and VOP₂ is greater than a predetermined value (step S11); VOP₂ is aVOP, the encoding processing of which is performed (step S12); and VOP₂is not a scene change VOP (step S13); so that the signal value of thescene change signal of VOP₃ is set to ‘1’ (step S14).

In addition, if the amount of change in information between VOP₄ andVOP₃ is not greater than a predetermined value, no scene change isdetected, and the signal value of the change amount detectioninformation is set to ‘0’. If VOP₄ is judged as being a VOP, theencoding processing of which is performed, as a result of code amountcontrol, the signal value of the skip signal is set to ‘0’. In thiscase, in accordance with the setting method described in the flow chartof FIG. 4, the amount of change in information between VOP₄ and VOP₃ issmaller than a predetermined value (step S11); VOP₃ is a VOP, theencoding processing of which is performed (step S19); and the signalvalue of the scene change signal of VOP₃ is ‘1’; so that the signalvalue of the scene change signal of VOP₄ is set to ‘1’ (step S21).

In a case as shown in FIG. 6 A, even if the change amount detectioninformation causes the encoding processing of VOP₃ where a scene changeis detected to be skipped, the scene change detection information can beapplied to perform the encoding processing by referencing the scenechange signal and the skip signal of VOP₃ for setting the scene changesignal of the next VOP₄ even when a skip VOP, the encoding processing ofwhich is not performed, and a single isolated scene change VOP occurconcurrently.

Next, FIG. 6B shows an example in which a scene change is detected insuccessive VOPs. In FIG. 6B, the amount of change between VOP₂ and thepreceding VOP₁ is greater than a predetermined value. In this case, thechange amount detection information is set to ‘1’, which indicates thatthe amount is greater than the predetermined value. Because the amountof change in information between VOP₂ and VOP₁ is greater than apredetermined value (step S11), VOP₁ is a VOP, the encoding processingof which is performed (step S12), and VOP₁ is not a scene change VOP(step S13), the scene change signal of VOP₂ is set to ‘1’, whichindicates that a scene change is detected (step S14). If VOP₂ is judgedas being a VOP, the encoding processing of which is performed, as aresult of code amount control, the signal value of the skip signal ofVOP₂ becomes ‘0’. The skip signals of all the subsequent VOPs are set to‘0’.

Because the amount of change in information between VOP₃ and VOP₂ isgreater than a predetermined value, a scene change is detected, and thesignal value of the change amount detection information is set to ‘1’.In accordance with the setting method described in the flow chart ofFIG. 4, the amount of change in information between VOP₃ and VOP₂ isgreater than a predetermined value (step S11); VOP₂ is a VOP, theencoding processing of which is performed (step S12); and VOP₂ is asingle isolated scene change VOP (step S13); so that the signal value ofthe scene change signal of VOP₃ is set to ‘2’, which indicates that theVOP is a continued scene change VOP (step S15).

In addition, because the amount of change in information between VOP₄and VOP₃ is greater than a predetermined value, a scene change isdetected, and the signal value of change amount detection information isset to ‘1’. In accordance with the setting method described in the flowchart of FIG. 4, the amount of change in information between VOP₄ andVOP₃ is greater than a predetermined value (step S11); VOP₃ is a VOP,the encoding processing of which is performed (step S12); and VOP₃ is acontinued scene change VOP (step S13); so that the signal value of thescene change signal of VOP₄ is set to ‘2’, which indicates that the VOPis a continued scene change VOP (step S15).

When successive scene changes occur as in from VOP₂ to VOP₄ describedabove, appropriate code amount control can be performed by setting thesignal value of the scene change signal of just VOP₂ to ‘1’ andproviding a mode for detecting the scene change signals of VOP₃ and VOP₄including successive scene changes as not being a single isolated scenechange but as being a continued scene change.

A scene change detected in a single isolated VOP is differentiated froma scene change detected in successive VOPs, the code amount assigned tothe latter is set to a smaller value than the code amount assigned tothe former, so that an excessive code amount will not be assigned to thesuccessive scene changes. Accordingly, degradation in image quality of apart other than the scene change and a drop frame can be avoided, and astable image quality can be obtained as a whole.

The amount of change between the current VOP and the previous VOP isdetected in the embodiment described above, but the amount of changebetween the current VOP and a VOP subsequent to the current VOP and theamount of change between the previous VOP and the current VOP may bedetected, and a scene change may be detected from these two amounts ofchange. In addition, the current VOP may reference a plurality of VOPs.

The change amount detector 102 of the embodiment described above detectsa scene change by obtaining the amount of change in information betweensuccessive VOPs taken in the frame memory 101, as the sum of theabsolute values of differences among pixels with regard to the luminancesignal, and comparing the amount of change with a predetermined value,but a chrominance signal may be used instead of the luminance signal.

In addition, the amount of change in information between successive VOPsis obtained as the sum of the absolute values of differences amongpixels, but a characteristic value of the picture such as the maximumvalue, minimum value, or central value of the signal value in thepicture can be used instead.

Further, the amount of change in information between successive VOPs iscalculated across the entire picture plane, but the picture plane canalso be divided into some small areas, and calculation and comparisoncan be made in the divided small areas.

A scene change may be detected by using a set of several scene changedetection methods described above. For instance, a set of the scenechange detection methods are used, and a scene change may be detectedwhen the condition of the scene change is satisfied in at least onemethod of the set of several scene change detection methods. The presentinvention further includes the other methods based on the set of thescene change detection methods described above, such as a method ofdetecting a scene change from the results of the detection methods basedon majority rule, a method of detecting a scene change based on majorityrule, with a weight assigned to the set of the detection methods, amethod of detecting a scene change when the condition of the scenechange is satisfied by a predetermined number of detection methods ormore, and a method of detecting a scene change when the condition of thescene change is satisfied by a specific method.

The image encoding apparatus according to the present invention performsthe encoding processing in accordance with the quantization parameterset by the encoding controller, so that the encoding processing can beperformed by making use of the scene change detection information evenif a VOP, the encoding processing of which is not performed, is a VOPwhere a scene change is detected.

Further, when a scene change is detected in successive VOPs, appropriateencoding processing can be performed as a whole by assigning anappropriate value as the code amount.

1. An image encoding apparatus comprising: an encoder which performsencoding processing of input VOPs successively, thereby outputting a bitstream; a switch circuit which allows the encoding processing by theencoder to be skipped in a unit of each of the VOPs; an encodingcontroller which generates an i-th skip signal indicating whether or notthe encoding processing of an i-th VOP is to be skipped and outputs thei-th skip signal to the switch circuit, where i is an integer notsmaller than 1, the encoding controller generating a control signal forcontrolling a code amount after the encoding processing of the i-th VOPby the encoder using an i-th scene change signal of the i-th VOP andoutputting the control signal to the encoder; and a scene change signalgenerator which generates the i-th scene change signal to be input tothe encoding controller on the basis of information of the i-th VOP anda VOP input prior to the i-th VOP and an (i-1)-th skip signal indicatingwhether or not the encoding processing of the (i-1)-th VOP input aperiod of N VOPs before the i-th VOP is skipped, where N is an integernot smaller than
 1. 2. The image encoding apparatus according to claim1, further comprising: a code amount detector which outputs a codeamount signal indicating a code amount after the encoding processing ofthe (i-1)-th VOP by the encoder to the encoding controller; the encodingcontroller generating the i-th skip signal on the basis of the codeamount signal.
 3. The image encoding apparatus according to claim 2,wherein the encoding controller generates the control signal to be inputto the encoder on the basis of the i-th scene change signal, the codeamount signal, and the i-th skip signal.
 4. The image encoding apparatusaccording to claim 1, wherein the scene change signal generatorincludes: a change amount detector which detects an amount of change ininformation between the (i-1)-th VOP and the i-th VOP; a skip signalholder which stores the (i-1)-th skip signal; a VOP information holderwhich stores information based on a VOP input prior to the i-th VOP; anda scene change detector which generates the i-th scene change signal tobe input to the encoding controller; the scene change detectorgenerating the i-th scene change signal on the basis of the amount ofchange, the (i-1)-th skip signal, and the information of a VOP inputprior to the i-th VOP.
 5. The image encoding apparatus according toclaim 4, wherein: the information of a VOP input prior to the i-th VOPis an (i-1)-th scene change signal of the (i-1)-th VOP; and the VOPinformation holder is a scene change signal holder.
 6. The imageencoding apparatus according to claim 5, wherein if an amount of changein information between the (i-1)-th VOP and the i-th VOP is greater thana predetermined value, the i-th VOP is treated as a scene change VOP;and if the i-th VOP is not the scene change VOP and if the encodingprocessing of the (i-1)-th VOP is skipped, the scene change detectorequalizes the i-th scene change signal with the (i-1)-th scene changesignal.
 7. The image encoding apparatus according to claim 4, wherein:if an amount of change in information between the (i-1)-th VOP and thei-th VOP is greater than a predetermined value and if an amount ofchange in information between an (i-2)-th VOP and the (i-1)-th VOP isnot greater than a predetermined value, the i-th VOP is treated as asingle isolated scene change VOP; if an amount of change in informationbetween the (i-1)-th VOP and the i-th VOP is greater than apredetermined value and if an amount of change in information between an(i-2)-th VOP and the (i-1)-th VOP is greater than a predetermined value,the i-th VOP is treated as a continued scene change VOP; the informationof the VOP input prior to the i-th VOP contains the information of the(i-1)-th VOP, the (i-2)-th VOP input a period of N VOPs before the(i-1)-th VOP, and the (i-3)-th VOP input a period of N VOPs before the(i-2)-th VOP; and the scene change detector generates the i-th scenechange signal so that a code amount after the encoding processing of thei-th VOP by the encoder when the i-th VOP is a continued scene changeVOP becomes smaller than a code amount after the encoding processing ofthe i-th VOP by the encoder when the i-th VOP is a single isolated scenechange VOP.
 8. The image encoding apparatus according to claim 7,wherein: if an amount of change in information between an (i-1)-th VOPand an i-th VOP is not greater than a predetermined value, an i-th VOPis not treated as a scene change VOP; if an amount of change ininformation between an (i-1)-th VOP and an i-th VOP is greater than apredetermined value, an i-th VOP is treated as a scene change VOP; ifthe i-th VOP is a scene change VOP, if the encoding processing of the(i-1)-th VOP is skipped and if the (i-1)-th VOP is not a continued scenechange VOP, the scene change detector generates an i-th scene changesignal indicating that an i-th VOP is a single isolated scene changeVOP; and if the i-th VOP is a scene change VOP, if the encodingprocessing of the (i-1)-th VOP is skipped and if the (i-1)-th VOP is acontinued scene change VOP, the scene change detector generates an i-thscene change signal indicating that the i-th VOP is a continued scenechange VOP.
 9. The image encoding apparatus according to claim 6,wherein: if an amount of change in information between an (i-1)-th VOPand an i-th VOP is not greater than a predetermined value, an i-th VOPis not treated as a scene change VOP; if an amount of change ininformation between an (i-1)-th VOP and an i-th VOP is greater than apredetermined value, an i-th VOP is treated as a scene change VOP; ifthe i-th VOP is a scene change VOP, if the encoding processing of the(i-1)-th VOP is not skipped and if the (i-1)-th VOP is not a scenechange VOP, the scene change detector generates an i-th scene changesignal indicating that the i-th VOP is a single isolated scene changeVOP; and if the i-th VOP is a scene change VOP, if the encodingprocessing of the (i-1)-th VOP is not skipped and if the (i-1)-th VOP isa scene change VOP, the scene change detector generates an i-th scenechange signal indicating that the i-th VOP is a continued scene changeVOP.
 10. An image encoding method comprising the steps of: detecting anamount of change in information between an i-th VOP, where i is aninteger not smaller than 1, and an (i-1)-th VOP input a period of N VOPsbefore the i-th VOP, where N is an integer not smaller than 1;generating an i-th scene change signal of an i-th VOP on the basis ofthe information of the i-th VOP and a VOP input prior to the i-th VOPand the (i-1)-th skip signal indicating whether or not the encodingprocessing of the (i-1)-th VOP is skipped; generating an i-th skipsignal indicating whether or not the encoding processing of the i-th VOPis skipped; and determining a code amount after the encoding processingof the i-th VOP by using the i-th scene change signal and performing theencoding processing of the i-th VOP.
 11. The image encoding methodaccording to claim 10, further comprising the step of: detecting a codeamount after the encoding processing of an (i-1)-th VOP on the basis ofa code amount after the encoding processing of the (i-1)-th VOP, thei-th skip signal, and the i-th scene change signal.
 12. The imageencoding method according to claim 10, wherein: if an amount of changein information between an (i-1)-th VOP and an i-th VOP is greater than apredetermined value, an i-th VOP is treated as a scene change VOP; andif the i-th VOP is not the scene change VOP and if the encodingprocessing of the (i-1)-th VOP is skipped, the i-th scene change signalof the i-th VOP is equalized with the (i-1)-th scene change signalcorresponding to the (i-1)-th VOP.
 13. The image encoding methodaccording to claim 10, wherein: if an amount of change in informationbetween the (i-1)-th VOP and the i-th VOP is greater than apredetermined value and if an amount of change in information between an(i-2)-th VOP and the (i-1)-th VOP is not greater than a predeterminedvalue, the i-th VOP is treated as a single isolated scene change VOP; ifan amount of change in information between the (i-1)-th VOP and the i-thVOP is greater than a predetermined value and if an amount of change ininformation between an (i-2)-th VOP and the (i-1)-th VOP is greater thana predetermined value, the i-th VOP is treated as a continued scenechange VOP; and the information of the VOP input prior to the i-th VOPcontains the information of the (i-1)-th VOP, the (i-2)-th VOP input aperiod of N VOPs before the (i-1)-th VOP, and the (i-3)-th VOP input aperiod of N VOPs before the (i-2)-th VOP; the method further comprisingthe step of: generating the i-th scene change signal so that a codeamount after the encoding processing of the i-th VOP by the encoder whenthe i-th VOP is a continued scene change VOP becomes smaller than a codeamount after the encoding processing of the i-th VOP by the encoder whenthe i-th VOP is a single isolated scene change VOP.
 14. The imageencoding method according to claim 13, wherein: if an amount of changein information between an (i-1)-th VOP and an i-th VOP is not greaterthan a predetermined value, an i-th VOP is not treated as a scene changeVOP; if an amount of change in information between an (i-1)-th VOP andan i-th VOP is greater than a predetermined value, an i-th VOP istreated as a scene change VOP; if the i-th VOP is a scene change VOP, ifthe encoding processing of the (i-1)-th VOP is skipped and if the(i-1)-th VOP is not a continued scene change VOP, an i-th scene changesignal indicating that an i-th VOP is a single isolated scene change VOPis generated; and if the i-th VOP is a scene change VOP, if the encodingprocessing of the (i-1)-th VOP is skipped and if the (i-1)-th VOP is acontinued scene change VOP, an i-th scene change signal indicated thatthe i-th VOP is a continued scene change VOP is generated.
 15. The imageencoding method according to claim 10, wherein: if an amount of changein information between an (i-1)-th VOP and an i-th VOP is not greaterthan a predetermined value, an i-th VOP is not treated as a scene changeVOP; if an amount of change in information between an (i-1)-th VOP andan i-th VOP is greater than a predetermined value, an i-th VOP istreated as a scene change VOP; if the i-th VOP is a scene change VOP, ifthe encoding processing of the (i-1)-th VOP is not skipped and if the(i-1)-th VOP is not a scene change VOP, an i-th scene change signalindicating that the i-th VOP is a single isolated scene change VOP isgenerated; and if the i-th VOP is a scene change VOP, if the encodingprocessing of the (i-1)-th VOP is not skipped and if the (i-1)-th VOP isa scene change VOP, an i-th scene change signal indicating that the i-thVOP is a continued scene change VOP is generated.